Automatic volume control circuits



Dec. 29, 1936. H. A. sNow AUTOMATIC VOLUME CONTROL CIRCUITS Filed Oct. 14, 1952 fav/Wm INVENTOR HAR BY ATTORNEY Patented Dec. 29, 1936 UNl'i'E STES ET OFFICE Harold A. Snow, Mountain Lakes, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application October 14, 1932, Serial N0. 637,699

6 Claims.

My present invention relates to amplifier gain control circuits, and more particularly to an automatic arrangement adapted for use in connection with amplifiers.

One of the main objects of my present invention is to provide an improved type of automatic gain control system for low or high frequency amplifiers, which system is ofthe general type utilizing a linear electron discharge tube detector and a non-linear electron discharge control tube. Since a system of automatic amplier gain control should only function when the signal voltage applied to the detector reaches a certain desired value, and then function to prevent this detector voltage increasing above this value, the characteristic of such an automatic control should preferably be non-linear. Since it is desirable to make the characteristicy of the detector of a receiver as nearly linear as possible, it is therefore preferable to employ a non-linear electron discharge tube device between the detector and the amplifier to be controlled. In general, such a combination of a non-linear electron discharge control tube, furnishing grid bias to a radio frequency amplifier for controlling the gain thereof, and a detector, is known. However, in the present application kthere are disclosed various improvements in such known arrangements, and these improvements comprise, as stated heretofore, one of the main objects of this invention.

Another important object of the present invention is to provide a method of controlling the radio frequency gain in a radio receiver by utilizing a separate rectifier in parallel with the receiver detector to furnish the radio frequency control bias, thus permitting the use of a linear detector and a non-linear receiver, the electron discharge tube rectifier being so adjusted that no appreciable load is placed across the detector input, and the rectifier further being sufliciently eiiicient to eliminate the need of a direct current amplifier for the rectifier output.

Another object of the kpresent invention is to provide an automatic gain control arrangement for use in connection with a radio frequency amplifier of a radio receiver, a rectifier being utilized in parallel with the detector input for providing the control voltage for the amplifier to be controlled, the control voltage being applied between the radio frequency cathodes and ground.

Still another object of the present invention is to provide an automatic gain control system wherein a screen grid rectifier tube is arranged (Cl. Z50- 20) in parallel with the detector input of a receiver, control voltage for a radio frequency amplifier being supplied from the plate circuit of the screen grid rectifier.

And still other objects of the present invention tion itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing,

Fig. 1 diagrammatically shows a receiver embodying the present invention,

Fig. 2 shows a modification of the arrangement shown in Fig. 1 and Fig. 3 shows still another form of the invention.

Referring now to the accompanying drawing there is shown in Fig. l a conventional radio receiver wherein the automatic gain control system is utilized in connection with a radio frequency amplifier stage, the latter comprising an electron discharge tube l of the indirectly heated cathode type. It is to be understood, however, that the present system may be used in connection with amplifier stages of either radiov frequency, intermediate frequency, or audio frequency. For example, the tube l, which precedes the detector tube 2, may be included in the fio intermediate frequency amplifier of a Super` heterodyne receiver, or it may even be included in the audio frequency amplifier shown coupled, as at M, to the plate circuit of the detector tube 2.

In any case, the input electrodes of the tube I are arranged to be coupled to'a source of signal energy, as at M1, and it is to be clearly under-v tion and operation of the arrangement including the amplifier tube I, the detector tube E, and the automatic volume control rectifier 3. It is to be clearly understood, however, that the control output of the rectifier tube 3 may be utilized for not only controlling the gain of the stage including tube I, but the gain of other amplifier stages preceding, or succeeding, tube I. Further, the gain of amplifier tubev I may be differentially controlled with respect to other amplifier tubes as by differential biasing of the different' radio frequency grids from the anode circuit of tube 3 as disclosed in my copending application Serial No. 638,323, filed October 18, 1932.

A single voltage supply unit is employed for energizing the various electrodes of tubes I, 2 and 3, and this unit preferably comprises the potentiometer included between the terminals +B' and -B. Those skilled in the art are well acquaintedwith the fact that these two terminals may be connected to a direct current power line, or the output or" a filtered rectified alternating current source. The anode of the Vdetector tube is connected by a lead 4, and through the primary coil of thecoupling transformer'M, to the terminal +B, while the cathode of the detector tube is connectedto the grounded conductor 5 through they bias resistor R1 which functions `to supply bias for the grid of tube 2 in alinear fashion with signal input. The linear detector tube 2 is arranged Vto have a xed bias applied toits grid through a path which includes the resistor R3, having a magnitude of about-2 megohms, andthe lead 6, the lead (i being connected to point a on the potentiometer. Thus, it will be Vseen that the bias on the grid of detector Vtube2 consists of a fixed bias derived from point a plus the variable bias across .the resistor R1. The Ypoint on the Voltage supply potentiometer designatedbyA the symbol OV., is maintained at ground potential, the conductor 5 being connected toysaid point at one end, and being grounded at its other end.

The amplier tube I Yhas its control electrode connected through the secondarycoilV of the coupling transformer M1, and through a lead 1, to the anode of the control rectier 3. 'I'he variable tuning condenser 8 has its rotor plates connected to the grounded conductor 5, it being noted that by means of a mechanical uni-control n 9, shown in dotted lines, the rotor plates of condenser 8 may be Varied simultaneously with the grounded *rotor plates of the variable tuning condenser, Iii which is arranged in the input circuit of detector tube The cathodeof tube i is grounded, and the anode of the tube is connected, through the primary coil II of the coupling transformer M2, to a point of positive potential on the voltage supply potentiometer. The actual connectionrof theampliiier anode to the supply potentiometer is not shown, it being believed that those skilled in the art will readily know to which point of the potentiometer the anode is to be connected. The grid of the detector tube 2 is connected to the high potential sideV of the secondary coil I2 of the transformer M2 through a coupling condenser I3, the low potential side of coil i2 being grounded.

The grid of the. control rectifier 3 is connected to po-int b in the grid circuit of the detector tube 2 through a path which includes the lead i4 and the fixed condenser i5. The `cathode of rectifier 3 is. connected by a lead If to point c on the voltage supply potentiometer, while the; grid of tube 3 is additionally arranged for slidable adjustment between point a and terminal -B of the voltage supply potentiometer by means of a lead it including a resistor R4, having a magnitude of about 2 megohms, the lead I6 terminating at the potentiometer in an adjustable manual volume control slider I'I. It will be noted that the other terminal of lead I5 terminates at a point between the condenser I5 and the grid of tube 3.

The slider II, which may be adjusted between point a and the -B terminal of the voltage supply potentiometer in the direction of the arrows, operates to adjust the control level of the automatic gain system, and also, operates as a manual volume control instrumentality. The anode of rectifier tube 3 is connected to a point on the Voltage supply potentiometer which represents a voltage of 4.5 volts through a lead I8 and resistor Rz, it being noted that a negative bias of 4.5 Volts is arranged in series with the potential drop obtainable across the resistor R2 which has a magnitude of about 100,000 ohms. It will be observed that the resistor Rz functions as the' control resistor for the bias of the radio frequency stage. It will be noted that all grid, cathode and anode leads are by-passed to ground for radio frequencycurrents through appropriate fixed condensers C. f

It is believed that the operation of the present arrangement Will be obvious from the foregoing description andFig. l of the drawing. The system employs as the automatic volume control tube a rectiiierl 3 which is arranged in parallel with the detector input. The detector itself functions by a linear manner in virtue of the po'- tential drop across the resistorv R1 which varies linearly with variation of' signal input tothe detector. non-linear manner, and the signal input energy impressed between thecontrol electrode and cathode of rectifier 3 creates a varying potential dropacross the resistor R2 which furnishes the control voltage for the amplifier tube I As has been stated before the fixed negativebias of 4.5 volts is arranged in series with the resistor R2.

Additional bias is furnished by the resistor R2 when the signal `voltage impressed lacross the input electrodes of rectifier tubeo3 is high enough for it to pass plate current through the resistor R2. Thus, when for one reason or another the carrier input to the detector tube 2 increases above the desired level which results in the predetermined reproduction volume level, the input to the rectifier 3 simultaneously increases, the potential drop across the resistor R2 increases, and the negative bias applied to the grid of amplier tube I increases, the radio frequency amplification of the increasing carrierY thus being decreased. This results in a maintenance of the reproduction volume level at a substantially constant value.

Obviously when the carrier intensity diminishes below the desired level the inverse action .will take place, and the potential drop across the resistorR2 will diminishv sufficiently to permit the amplifier I to increase the amplification of the incoming signal energy to the desired volume level. -As stated heretofore, proper adjustment of the slider I1 not only controls volume manually, but also functions to adjust the desired level at whichk the automatic gain control system will operate. K l

In Fig. l the control bias has beenshown as applied to the normally negative signal grid of tube I and the cathode being at ground po- However, the rectifier 3 operates ina` 'drop across the platel to cathode pathv ci the tential. A modification of this arrangement is shownv in Fig. 2, wherein the control bias is applied to the cathode of tube I and the signal grid being at ground potential. Otherwise, the arrangement shown in Fig. 2 is similar to that shown in Fig.,1. In order to have the detector tube 2 function satisfactorily as a linear rectier, the grid of detector tube 2v should have a fixed negative bias of between -20 to -30 volts applied thereto, with a plate voltage on the detector of between 135 to 180 volts. It will be noted that the control bias resistor R2 is connected in this modiiication between the cathode of tube 3 and a point on the voltage supply potentiometer, which is omitted to preserve simplicity of description, having a value of 4.5 volts above ground.

The cathode of amplifier tube I is, then, connectedthrough-a lead I to the cathode side of resistor R2". I-Iere, again, the slider I'I, connected in the grid circuit of rectifier tube 3, is arranged for manual volume control. The radio frequency-amplifier cathode is at +45 volts with respect to ground, plus the drop across resistance Re which is controlled by the plate current of the control tube 3. The resistor R2 should be of a low resistance since the Ip of the radio frequency amplifier tubes iidws through this resistance.

The operation of the modification shown in Fig. 2 is believed obvious in view o f the description shown in Fig. 1. The amplification eii'iciency oftube I is varied by controlling the potential of the cathode thereof with respect to its grounded grid, and the varying drop across the resistor R2 is employed for this purpose.

In Fig. 3 is shown a modication of the invention wherein the rectifier in parallel with the input of the detector tube 2 is of the screen grid type, and is designated by the numeral 3. The control resistor R2 is connected between the anode of tube 3' and the grounded conductor 5', while the resistor Re is a leak resistor, having a magnitude of about 1 or 2 megohms. The resistor Rn is shown connected between the grid circuit of tube 3 and the terminal -B of the voltage supply potentiometer. The grid of detector tube 2 is connected through the coupling condenser I5, having a magnitude of about 0.0001 to 0.0005 microfarad, to the grid of rectifier tube 3'.

The shield, or screen, grid of tube 3 may be connected by a lead 30 to a point generally designated by the reference letter X. This point specically may be any of the points X1, X2 and X3. The cathode of control tube 3' is connected by a lead 3l, terminating in an adjustable slider Il', to a point about I0 volts above the terminal -B, and as pointed out above the slider Il functions to facilitate manual setting of the input voltage level.

The arrangement shown in Fig. 3 functions in a manner similar to that shown in Fig. l. The signal input to thev detector tube 2 is, also, impressed between the grid and cathode of rectifier 3', and the potential drop across the resistor R1 is utilized for providing the control bias for the radio frequency grids. The general showing of the manner of connecting the screen grid of tube 3 to the three points X1, X2 and Xn shows the flexibility of this circuit. The system shown in Fig. 3 can be used either in conjunction with the detector as shown, o-r with the input of any amplifier stage using the potential amplier to bias preceding amplifier stages, andi it is to be clearly understod that it can be thus employed either with radio or audio frequency systems.

While YI have indicated and described several systems for carrying my invention into eiTect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention asset forth in the appended claims.

What I claim is:

1. In combination with a radio frequency amplifier whose gain is to be automatically controlled, a detector including at least a cathode, anode and signal grid, an impedance in the detector space current circuit providing a signalcontrolled negative bias for the signal grid, means for coupling the output circuit ci the detector to an audio frequency amplifier, a rectifier, means for connecting the input circuit of said detector to the input electrodes of said rectifier, an impedance in the anode circuit of said rectifier, a connection between one side of said impedance and an input electrode of said controlled amplifier, a common voltage supply potentiometer for energizing the electrode circuits of said controlled amplier, detector and rectifier,v means, comprising a portion of said potentiometer in series with said space current impedance, for providing an invariable negative bias for the detector signal grid, and a manual volume control device connected between an input electrode of said rectifier and a predetermined point on said voltage supply potentiometer.

2. In combination with a radio frequency amplifier whose gain is to be automatically controlled, a detector including at least a cathode, anode and signal grid, an impedance in the detector space current circuit providing a signalcontrolled negative bias for the signal grid, means for coupling the output circuit of the detector to an audio frequency amplier, a rectier, means for connecting the input circuit of said detector to the input electrodes of said rectier, an impedance in the anode circuit of said rectier, a connection between one side of said impedance and an input electrode of said controlled amplifier, a common voltage supply potentiometer for energizing the electrode circuits of said controlled amplifier, detector and rectifier, and means, comprising a portion of said potentiometer in series withv said space current impedance, for providing an invariable negative bias for the ydetector signal grid.

3. In combination with a radio frequency amplifier whose gain is to be automatically controlled, a detector including at least a cathode, anode and signal grid, means for coupling the output circuit of the detector to an audio frequency amplier, a rectier, means for connecting the input circuit of said detector to the input electrodes of said rectifier, an impedance in the anode circuit of said rectifier, a connection between one side of said impedance and an input electrode of said controlled amplifier, a common voltage supply potentiometer for energizing the electrode circuits of said controlled amplier, detector and rectifier, the cathode circuit of said detector including a resistor connected between the cathode of the detector and a point of said voltage supply potentiometer which is at ground potential, the Ypotential drop i across said cathode resistor varying substantially linearly with voltages impressed upon the detector input, a connection between the detector grid and a point on the potentiometer which is negative with respect to said ground point, and an adjustable connection between the rectier grid and a portion of the potentiometer connected to said negative point.

4. In combination witha radio frequency amplifier whose gain is to be automatically controlled, a detector including at least a cathode, anode and signal grid, an impedance in the detector space current circuit providing a signal controlled negative bias for the signal grid, means for coupling the output circuit of the detector to an audio frequency amplifier, a rectier, means for connecting the input circuit of said detector to the input electrodes of said rectiiier, an impedance in the cathode circuit of said rectifier, a connection between one side of said impedance and an input electrode of said'controlled amplier, a common Voltage supply potentiometer for energizing the electrode circuits of said controlled amplifier, detector and rectifier, said. controlled amplier having its control grid grounded, the detector grid being connected to a point of negative potential on the potentiometer, said connection between said controlled amplifier and said rectifier cathode impedance being connected between the cathode of said controlled amplier and said cathode impedance, said cathode impedance being connected to a point on the potentiometer which is positive with respect to ground.

5. In a radio receiving system which comprises a signal transmission tube and a rdemodulator tube which includes at least a cathode, a signal inputv electrode and an output electrode, means for impressing the output of the first tube; on the signal input electrode of the demodulator, a utilization network coupled to the demodulator output electrode, an impedance in the space current path of said demodulator adapted to provide a signal-controlled negative bias for the signal input electrode of the demodulator, a source of direct current voltage of a substantial conf stant magnitude in series with said impedance between the signal input electrode and cathode of said demodulator, and means responsive to Variations in the received signal amplitude for automatically varying the gain of said signal transmission tube in a sense to maintain the signal amplitude level at said demodulator substantially uniform. Y

6. In a radio receiving system which comprises a signal transmission tubeV and a demodulator tube which includes at least a cathode, a signal input electrode and an output electrode, means for impressing the output of the rst tube on the signal input electrode of the demodulator, a utilization network coupled .to the demodulator output electrode, animpedance in the space current path of said demodulator adapted to provide a signal-controlled negative-bias for the signal input electrode of the demodulator, a

source of direct current voltage of a substantial ,Y

constant magnitude in series with said imped- 

